Alarm system



March 10, 1942. c. Hi BACKUS `r-:T AL

ALARM SYSTEM Filed April 22, 1959 2 Sheets-Sheet l vvvvv INVENTORS GLY/)E H. BACK'l/S AND 7 ALF D M. DUNATO ATTORNEY March 10, 1942- c. H. BACKUS ET AL ALARM SYSTEM Filed April 22, 1939 2 Sheets-Sheet 2 INVENTORS CLYDE H. BAC/(US AND XQQQML. A

A i?? M. DONATO ATTORNEY.

Patented Mar. 10, 1942 ALARM SYSTEM Clyde H. Backus, Marion, and Alfred M. Donato,

Wareham, Mass., assignors to Radio Corporation of America, a corporation of Delaware Application April 22, 1939, Serial No. 269,332

(Cl. 17E-66) Claims.

The present invention relates to radio telegraph transmitters and, more particularly, to means for preventing damage thereto due to the transmission of long dashes.

Since a radio telegraph transmitter, even when operating, is not continuously radiating due to the presence of spacing intervals between code elements, letters and words, the present practice is to operate a telegraph transmitter at considerably greater power ratings than would be permissible for continuous radiation such as used in radio telephone service. This is perfectly satisfactory under normal circumstances but due to various reasons the keying circuit for the transmitter may occasionally be maintained in a closed position for longer periods of time than is desirable for long tube life.

An object, therefore, of the present invention is to prevent the transmission of long marking intervals in a transmitter constructed according to the principles described above.

A further object of the present invention is to provide means for giving an alarm when a radio telegraph transmitter, as heretofore described, is keyed for intervals above a predetermined maximum.

Still another object is to provide a means for shutting down a radio telegraph transmitter, as heretofore described, when a dash of greater than predetermined length is transmitted.

Brieiiy, the present invention provides a means connected to a radio telegraph transmitter for charging a condenser during the time of transmission of spacing intervals only and for discon-I necting the charging source during marking intervals. Means are provided across said condenser for continuously discharging the condenser at a slow rate. If the charging means remains disconnected from a condenser for a greater than predetermined time the charge leaks ofi the condenser and a cut-oliF bias is removed from the grid of a vacuum tube. The resultant plate current operates a relay to sound an alarm. The relay may also be arranged to break the plate circuit of the transmitter or to break one of the other interlock circuits associated with the transmitter.

Reference will now be had for a more complete understanding of our invention to the following detailed description which is accompanied by drawings in which Figure 1 shows schematically a device according to the present invention connected to a portion of a radio transmitter, Figure 2 shows a modication of the form of invention shown in Figure 1 and Figures 3 and 4 show further modifications of Figure 1 which may in some cases be desirable.

Referring, now, to Figure 1, reference numeral I indicates a keying unit which is connected to a frequency doubler stage 2, or a power amplifier stage or a radio transmitter. The frequency doubler stage 2 includes a thermionic discharge tube 3 having a source of carrier energy 5 connected to the input thereof and a tuned output circuit 6, The output circuit 6 is connected to further doubler or amplier stages as indicated by the arrow labeled output Plate energy is supplied from a source which, in the drawing, is indicated by +3000 V. Also fed through the resistor 4 is the keying stage I which may comprise a pair of vacuum tubes I and t connected in parallel. In some cases a single tube may be used. Filaments of vacuum tubes l and 8 are energized through a iilament transformer 9. The filament circuit of the vacuum tubes "I and 8 has a pair of serially connected resistors i0 and I0' connected thereacross in order to provide an electrical center tap which is ordinarily grounded. The grids of tubes 'i and 8 are connected in parallel and connected to the keying circuit II. During the spacing intervals when the key is up a positive potential is placed on the grids of tubes 'I and 8 from the source +200 v. The tubes therefore, draw a heavy plate current and causes a large voltage drop through resistance fl. This effectively reduces the plate potential supplied to tube 3 to such an extent that the resultant output is insuiiicient to energize the succeeding stages of the transmitter and thus no signal is radiated. When key I2 is closed a negative potential is applied to the grids of tubes l and 0 from the source -250 v., thus biasing the tubes to a point beyond cut-olf and substantially no current is drawn through resistor 4 by tubes 'I and 8. Under these conditions there is very little voltage drop through the resistor i and substantially the entire plate potential from +3000 v. is applied to tube 3, thus providing a maximum marking output. In accordance with my invention the connection from the midpoint between resistors Ill and I0' to ground is completed through a key click choke I3 and a resistor I 4. During the operation of the transmitter with the key up and tubes 'l and 8 drawing a heavy current there may be, to take a concrete example, approximately 200 milliamperes owing from the center tap of the filament through choke I3 and resistor I4 to ground. The voltage drop across resistor I4 as a result of the 200 milliamperes iiowing therethrough is suiiicient to hold relay I5 in its operated position, thus applying from a potential source 170 v. a negative potential to the ungrounded plate of condenser I'I. The resultant charge across condenser Il is sufficient to bias tube I8 beyond cut-ofi and no current flows in the plate circuit of tube I3. The relay I9 connected between the cathode of tube I8 and ground is, therefore, maintained in a released position. With the transmitter in a key down position for transmitting a marking impulse there is normally very little current flowing in the circuit including choke i3 and resistor I4. This allows relay I5 to open and the condenser I1 to gradually discharge through the resistors 2U and 2l. The bias on the grid of tube i8 is thus slowly released and when it becomes low enough the tube begins to draw plate current. The gradually increasing plate current, if the key I2 is held down long enough, actuates relay I9 thus applying a potential to the actuating circuit for bell 22 and giving an alarm. Simultaneously, the relay IG may be arranged to break an energizing circuit for relay 23 which is connected in series with the transmitter power supply or connected in one of the interlock circuits associated therewith. The de-energization of relay 23 opens the power supply circuit or the interlock circuit, as the case may be, and shuts down the transmitter. If desired, relay 23 may be made slow to release or a mechanical delay may be interposed so as to allow a short interval of time to elapse between the actuation of the alarm and disabling the transmitter. This arrangement will give the operator time to clear the trouble before the transmitter closes dov/n. Obviously, of course, the transmission of dots and short dashes, while actuating relay It., does not open the charging circ-uit for condenser Il long enough to allow the condenser to discharge completely enough to remove the negative bias on tube I3.

By varying the capacity ci condenser l? and/ or the resistance of the shunt composed of resistors and 2|, the discharge interval can be varied up to a minute or more but for most applications the discharge interval should be about four seconds.

In Figure 2 we have shown a modification of the device shown in Figure i in which the portion to the right of the dotted line in Figure 2 is supposed to be substituted for the portion to the right of the dotted line in Figure l.

In. this modification with the transmitter in the key-up or spacing position as heretofore assumed, the 290 milliamperes current iiow through resistor Ici causes a potential of about 3() volts to be applied to the plate and grid of tube 25. While tube 25 is shown with the grid and plate connected together, obviously, of course a two element rectifier may be used, if desired. Tube 26 acting as a rectier permits condenser 2'! to charge and also current to iow through the resistor 3! thus holding a blocking bias on tube 23. As tube 23 is biased well beyond cut-off no current flows in this plate circuit and relay Il remains unactuated. With the key down and the transmitter radiating a marking impulse, there is, as previously explained, very little current ilowing through resistor ill. rThis removes the 30 volt charging source for condenser 2'! which therefore discharges through resistor iii. Tube 26 prevents a rapid discharge through resistor 24 as its cathode is now positive with respect to its plate. When the condenser 2l has discharged to below the voltage required to block tube 28,

plate current flows through relay I 9 and, as described with respect to Figure 1, bell 22 is actuated and relay 23 in the plate circuit of the transmitter is die-energized thus shutting down the transmitter.

The modification shown in Figure 2 using a rectifier tube for charging a condenser instead of a relay, as shown in Figure l, results in higher permissible keying speeds and eliminates one moving and expensive part but necessitates the grid of tube 28 instead of the cathode being grounded in order to obtain proper grid polarity. This requires an ungrounded plate supply for tube 28. A separate plate supply source 32 must therefore be used in this modication instead of the regular transmitter power supply. However, operation with the separate power supply may, in some cases, be considered somewhat better as regulation due to keying is eliminated and the entire unit is sell-contained. li desired, the resistor 3l may be a potentiometer with the movable arm connected to the cathode of tube 28 instead of using a fixed resistor. With this type of construction the delay period may be easily adjusted by moving the movable arm oi the potentiometer.

In Figure 3 we have shown a modification in which the plate current rlow in tube 3 is used to actuate the alarm. Resistor Eil is placed in the cathode circuit of tube 3 and relay l5 is substituted by relay which has a normally closed Contact. With 'this modification relay stands unactuated during spacing intervals, and permits condenser H to charge as previously described. During mar :ing intervals relay 55 is actuated, thus allowing condenser il to discharge as before.

The circuit shown in Figure el. be substituted for the portion of Figure 3 to the right oi dotted line Y-Y. With the transmitter in the key-up position battery :t2 biases tube 28 beyond cut-01T so that no current flows in the plate circuit of tube 28 and relay I9 stays in the back or idle position. l/Vith the transmitter in the keydown position current iiows in resistor Till causing a positive potential to appear at the ungrounded end. This positive potential causes current to low in resistor part of which charges condenser 21 and part goes to ground through resistor 3 l. Values of resistors Il?, and 3i are chosef; .such that condenser 2T slowly charges up and changes the bias on the grid or tube 23 in the positive direction until tube becomes conductive. Relay it* is then actuated, since its coil is in the plate circuit of tube When the transmitter goes to the key-up position any positive charge on condenser Si rapidly discharges through tube 25 and resistor 2li. Resistor 24 being of a low value in comparison to resistor 43 this discharge is very rapid. The discharge of condenser 3l allows the bias on the grid of tube 2B to swing negative beyond cut-ofi thus allowing relay I9 to go to its back or idle position.

While we have shown and particularly described several embodiments of our invention, it is to be distinctly understood that our invention is not limited thereto but that modications within the scope of our invention may be made.

We claim:

l. In a radio telegraph transmitter, a keying circuit for said transmitter, said circuit including a resistor which carries current during spacing intervals only, means responsive to a voltage drop through said resistor for charging a condenser in short pulses, means for continuously discharging said condenser at a slow rate, a thermionic discharge tube having an anode, a cathode and a grid, a source of potential and a relay connected between said anode and cathode, said grid and cathode being so connected to said condenser that the charge in said condenser biases said tube beyond cut-off, and means responsive to an energization of said relay for actuating an alarm and disabling said transmitter, each single pulse being suiiicient to charge said condenser beyond the cut-off potential of said tube.

2. In a radio telegraph transmitter, a keying circuit for said transmitter, said circuit including a resistor which carries current during spacing intervals only, a relay having a winding energized by the potential drop through said resistor, a condenser, a source of potential, said relay when energized connecting said source to said condenser, each energization of said relay charging said condenser to a potential substantially equal to that of said source, means for continuously discharging `said condenser at a slow rate, and means responsive to a diminution of the charge in said condenser below a predetermined value for actuating an alarm.

3. In a radio telegraph transmitter, a keying circuit for said transmitter, said circuit including a resistor which carries current during spacing intervals only, a relay having a winding energized by the potential drop through said resistor, a condenser, a source of potential, said relay when energized connecting said source to said condenser each energization of said relay charging said condenser to a potential substantially equal to that of said source, means for continuously discharging said condenser at a slow rate, and means responsive to a diminution of the charge in said condenser below a predetermined value for actuating an alarm and disabling said transmitter.

4. In a radio telegraph transmitter, a keying circuit for said transmitter, said circuit including a. resistor which carries current during spacing intervals only, a relay having a winding energized by the potential drop through said resistor, a condenser, a source cf potential, said relay when energized connecting said source to said condenser, means for continuously discharging said condenser at a slow rate, a thermionic discharge tube having an anode, a cathode and a grid, a source of potential and a relay connected between said anode and cathode, said grid and cathode being so connected to said condenser that the charge in said condenser biases said tube beyond cut-ofi, said tube being responsive to a diminution of the charge in said condenser below a predetermined value to actuate said relay, and means responsive to an actuation of said relay for operating an alarm, each operation of said relay charging said condenser beyond the cut-off potential of said tube.

5. In a radio telegraph transmitter, a keying circuit for said transmitter, said circuit including a resistor which carries current during spacing intervals only, a condenser having one terminal connected to one end of said resistor, a rectifier so connected between the other terminal of said condenser and the other end of said resistor that a voltage drop through said resistor charges said condenser, means for continuously discharging said condenser at a slow rate, a therrnionic discharge tube having an anode, a cathode and a grid, a source of potential and a relay connected between said anode and cathode, said grid and cathode being so connected to said condenser that the charge in said condenser biases said tube beyond cut-off, said tube being responsive to a diminution of the charge in said condenser below a predetermined value to actuate said relay, and means responsive to an actuation of said relay for operating an alarm, the charge on said condenser due to each single spacing interval being sufficient to bias said tube beyond cut-off.

CLYDE H. BACKUS. ALFRED M. DONATO. 

